Support is requested for an International Workshop on Time-of-Flight Emission Tomography to be conducted at the Washington University of Medicine in the Spring of 1982. At this institution, a multidisciplinary collaboration between the Departments of Radiology, Medicine, Neurology and the Biomedical Computer Laboratory is reflected in NIH support for emission tomography through NHLBI, NINCDS, and DRR. Verbal contact has established the programmatic relevance for all three. Accordingly, we propose cooperative inter-Institute support via those agencies. The acquisition and use of differential time-of-flight information presents a number of technical challenges, but the potential benefits in improved temporal and spatial resolution as well as in lower radiation doses to patients clearly justify current efforts. The technical challenges include the development of fast photon-detector systems, the design and implementation of circuits for resolving coincidences in the range of hundreds of picoseconds, the derivation of new algorithms for incorporating the time-of-flight data in tomographic reconstructions, and the definition of efficient processing structures for acquiring and manipulating the data. Groups in Japan, France, and the United States have all made progress toward implementation of prototypical time-of-flight systems. Important to the adequacy of systems now under development are the requirements for application of the technology. The field is now at an important juncture from which future directions should be established through dialogues among system developers and biomedical scientists who are sensitive to research needs.